Milling Operations downhole generate cuttings that a captured in tools associated with a mill frequently referred to in the industry as junk catchers. There are many configurations for such tools. Some have external seals that direct cuttings coming up from a mill around the outside of the tool back into the tool so that the circulating fluid can exit while the debris is captured in the tool body. Examples of this design are U.S. Pat. Nos. 6,176,311 and 6,607,031. Another design involves establishing a reverse circulation with jets that discharge outside a tool body toward a mill below and act as eductors to draw fluids through the mill and into a screened section central passage. Once the debris laden fluid exits the central passage the velocity slows and debris drops into an annular passage and the fluid keeps going toward the top of the tool. On the way out the top the remaining debris is left on a screen and can drop into the same annular space that caught the larger debris further down the tool as the now screened fluid is drawn by the jets at the top of the tool to go right back down around the outside of the tool toward the mill so that the cycle can repeat.
FIG. 1 illustrates the basics of this known design. A mill 10 generates cuttings that are removed with reverse circulation that goes up passage 12 and exits at 14 into a wide spot 16 in the tool body 18. The heavier debris falls into annular space 20 around the passage 12 while the fluid stream with some smaller debris continues up the tool body 18 until it reaches a screen 22. The debris remaining is caught outside the screen 22 and eventually falls to annular space 20. The clean fluid is drawn by the jets 24 fed by fluid pumped from the surface through a string (not shown). Exhaust from the jets 24 combined with fluid drawn by those jets now goes back down around the tool body 18 toward mill 10 and the rest goes up to the surface outside the tubular string that runs from the surface (not shown).
FIG. 2 shows a detail of the junk catcher of FIG. 1. What is depicted is the lower end just above the mill 10. A threaded connection 26 holds the bottom sub 28 to the tool body 18. Debris 30 typically falls down in annular space 20 and wedges tube 32 that defines the passage 12 and prevents the ability to relatively rotate the bottom sub 28 with respect to body 18 to get the threaded connection 26 to let loose. That threaded connection 26 has to get undone so that the debris 30 can get flushed out of the tool when it is brought to the surface. Note that the tube 32 is attached to the bottom sub 28 and in the past efforts to get the threaded connection undone have sheared the tube 32 or have otherwise caused it to crack or fail when debris 30 got compacted in annular space 20.
Another issue was that tube 32 was prefabricated to a predetermined length which limited the volume of the annular space 20. Yet another issue occurred when the surface pumps were shut off and debris on the screen 22 can fall through the hat 34 through the side openings 36 under it.
Turning now to FIG. 7, a detailed view of the mill 10 from FIG. 1 is shown with a central passage 38 leading to circulation outlets 40 four of which can be seen in the associated bottom view. Passages 40 are far smaller than passage 38 that feeds them. This layout worked well for normal downhole milling with circulation going down passage 38 to outlets 40 when a tool or other wellbore obstruction was milled out in a traditional way. However, in conjunction with the debris catcher shown in FIG. 1 there was a problem since the circulation patterns are reversed for the debris catcher in FIG. 1 and cuttings are reverse circulated into the body of mill 10 which leads to plugging of the passages 40. The mills of FIG. 7 had blades 42 featuring inserts 44 and textured carbide faces in between to assist in the milling operation.
The present invention provides for greater capacity variation for the tool illustrated in FIG. 1 leading to a modular design with passages that feature dog legs to promote dropping of debris into annular catch volumes located below dog legs. An alternative uses a modular approach with aligned modules that have flapper valves that can fall shut when circulation stops to prevent debris from falling back to the mill. The mill configuration has been changed to accommodate reverse circulation without the plugging issues of prior designs illustrated in FIG. 7. These and other aspects of the present invention will be more apparent to those skilled in the art from a review of the description of the preferred embodiments and associated drawings that appear below while understanding that the full scope of the invention is given by the claims.